In the continuous casting method of manufacturing steel, molten metal is cast directly into thin strip by a casting machine. The shape of the thin cast strip is determined by the mold of the casting rolls used in the machine. The cast strip may be subjected to cooling and processing upon exit from the casting rolls.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated laterally positioned casting rolls which are internally cooled, so that metal shells solidify on the moving casting roll surfaces and are brought together at the nip between the casting rolls to produce a thin cast strip product delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip, to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the casting rolls so as to restrain the two ends of the casting pool.
Typically, one of the casting rolls is mounted in fixed journals, and the other casting roll is rotatably mounted on supports that can move against the action of a biasing force to enable the roll to move laterally to accommodate fluctuations in casting roll separation and strip thickness. The biasing force may be provided by helical compression springs or alternatively, may comprise a pair of pressure fluid cylinder units.
A strip caster with spring biasing of the laterally movement of one casting roll relative to another casting roll is disclosed in U.S. Pat. No. 6,167,943 to Fish et al. In that apparatus, the biasing springs act between the roll carriers and a pair of thrust reaction structures, the positions of which can be set by operation of a pair of powered mechanical jacks to enable the initial compression of the springs to be adjusted to set initial compression forces which are equal at both ends of the casting roll. The positions of the roll carriers need to be set and subsequently adjusted after commencement of casting, so that the gap between the rolls is maintained across the width of the nip in order to produce a strip of stable profile. However, as casting continues, the profile of the strip will inevitably vary due to eccentricities in the rolls and dynamic changes due to variable heat expansion and other dynamic effects.
Eccentricities in the casting rolls can lead to strip thickness variations along the strip length. Such eccentricities can arise either due to machining and assembly of the casting rolls, or due to distortion of the hot casting rolls during the casting campaign due to non-uniform heat flux distribution. Specifically, each revolution of the casting rolls will produce a pattern of thickness variations dependent on eccentricities in the casting rolls, and this pattern in the strip will be repeated with each revolution of the casting rolls. Such repeating pattern periodically with roll rotation will be sinusoidal, but there are secondary or other vibrational fluctuations which are not of sinusoidal patterns directly related to the rotation speed of the casting rolls.
With improvements in the design of the casting rolls for a twin roll caster, particularly by the provision of textured surfaces which enable control of the heat flux at the interface between the casting rolls and the casting pool, it has been possible to achieve dramatic increases in strip casting speeds. However, with casting of thin strip at higher casting speeds, there has been an increased tendency to produce both high frequency and low frequency vibrations in the system that can affect the quality of the cast strip.
The high frequency variations or defects in the cast steel strip may be due to high-frequency chatter, medium frequency chatter, and brush-derived chatter in the twin caster assembly. The low frequency gauge variations may be defects known as herringbone (a type of strip defect that manifests itself at specific low frequencies), white lines (another type of defect at low frequencies), and twice-per-roll revolution related force fluctuations, which may also be due to unwanted low frequency vibrations in the caster assembly. Other types of defects have also been observed.
U.S. Pat. No. 6,604,569 to Nikolovski et al. describes how varying the speed of rotation of the caster rolls can be performed to reduce, if not eliminate, certain variations in the cast steel strip. For example, the repeated thickness variations from eccentricities in the casting rolls can be reduced by imposing a pattern of speed variations in the speed of rotation of the rolls. Compensation in this manner is possible because even small speed variations in casting speed can be effective. The Nikolovski patent relies on measurement of the thickness of the steel strip after it is produced to determine what the compensation in the speed of the rolls should be for variations in strip thickness with roll eccentricity. However, measurement of the thickness of the thin cast steel strip is not a direct indication of what is happening at the casting rolls, and does not compensate for the high frequency and low frequency vibration that may be occurring in the thin cast steel strip system.
U.S. Pat. No. 5,927,375 to Damasse et al. describes measuring the roll separating force at the casting rolls of a twin roll casting system, and observing at periodic harmonic frequencies associated with rotation of the casting rolls. The Damasse device controls for casting roll eccentricity due to casting roll shape, and nothing else. Damasse et al. does not measure or correct strip profile eccentricities unrelated to casting roll eccentricity and roll rotation, Strip profile defects can be unrelated to casting roll shape and roll rotation, which can occur because the heat flux on each casting roll can change and for other dynamics and vibrations encountered by the caster system.
Identifying and correcting the various defects that can occur in the thin cast strip profile, and to do that in real time during the casting campaign, would be beneficial in providing quantity strip. The accurate varying in real time of the roll separation gap, generally on the order of a few millimeters or less, to define an appropriate separation of the casting rolls at the nip in response to identified strip defects is needed. Adjusting the gap between the casting rolls by adjusting the biasing force against which the casting rolls move during the casting campaign also accommodates for fluctuations in strip thickness, notably during start up. In addition, adjusting casting speed and casting pool height in real time in response to identified strip defects could improve the quality of the thin cast strip.